Illuminating deep weathering and weakness zones in the Himalayas

Introduction and case study

The construction of a new tunnel usually requires a detailed feasibility study. The purpose of such study is to plan and prepare for construction, including the support installation, the possible stability problems, and more. Not conducting these investigations may lead to significant delays and increased costs, as experienced by the numerous projects where large and unforeseen problems occurred during construction. Normally, the expenditures for engineering geological investigations and site characterization should be at least 3-5% of the estimated project cost. Recent advances in geoscience have however given us new tools to investigate the rock mass quality at different scales. The application of new tools with traditional approaches may help in a better planning of the tunnel construction. Practical examples indicate that small investments at the beginning of a project could save a lot later in the project (and in the longer term). This paper describes some of these methods, and in particular geophysical approaches and some numerical codes mainly taken from Bhasin et al.(2016). For each method, a theoretical description is proposed followed by a practical example and a few results from a selected case study.

The case study involves the planning of a new road tunnel in the Bhutan Himalaya between the capital, Thimpu, and Wangdue. Both cities are fast growing areas in Bhutan and the existing 70 km long road between Thimpu and Wangdue is not well suited for this increased traffic. The road is steep, winding, narrow, and goes over the Dochu La pass at about 3100 m. In addition, there exists ice and snow in winter, and unstable road cuttings and slopes during the rainy season, which have caused repeated problems. The improvement of the old road is not considered relevant and a new road link was proposed. This new road would reduce the traveling distance by 36 km. It would, depending on the options, include either a 10.5 or 14.5km long tunnel. The construction of the tunnel, including the choice of the most favorable route and the design of the rock support, was studied using a broad range of methods, from geological surveys to geophysical investigations, including also numerical simulations and cost analyses.

Conclusion

The Norwegian Geotechnical Institute (NGI) has been involved in several studies over the past two decades for constructing underground structures in the Himalaya and has developed to this respect special competence in assessing rock mass quality using the latest geosciences advances. Recently, a detailed feasibility study was performed for a new road tunnel in the Bhutan Himalaya. The study included engineering geological mapping, rock mass characterization, geophysical investigations and numerical modelling for verifying the rock support requirements in the tunnel. Advanced airborne electromagnetic AEM surveys were performed along the tunnel corridor to provide information on the rock mass quality along the potential tunnel alignment and for visualizing of the existing sub-surface geological conditions. Specifically, high resistivity areas i.e. competent bedrock was distinguished from low resistivity areas i.e. incompetent or weathered rock. The rock reinforcement requirements estimated from the Q-system of rock mass classification were verified through both finite element and distinct element modelling. This paper illustrated the various studies performed along the proposed alignment for gaining an insight into the prevailing rock mass conditions at the proposed site. In the Himalaya, which is generally characterized by steep slopes, lofty hills, and complex geological and tectonic settings, such studies are warranted for planning new and upcoming underground projects. It is believed that the combination of the different approaches, which have been described in this paper, may help in a better planning of the tunnel construction in the Himalaya leading to significant cost savings in the long-term.

Acknowledgements

The authors are thankful to the Department of Geology and Mines (DGM) in Thimpu, Bhutan and their NGI colleagues (Andreas Pfaffhuber and Maarten Vanneste) for their technical support during the investigations. Dr. R.K. Goel, Chief Scientist, CSIR-CIMFR Roorkee, India is thanked for his valuable suggestions and comments. The Norwegian Agency for Development (NORAD, Senior Advisor Harald Birkeland) is thanked for its financial support to the institutional co-operation project between DGM and NGI.

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Reference

The full paper can be requested through the download link above or found directly at www.isrmtt.com:

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Bhasin, R., Pabst, T., Bazin, S., and Aarset, A. 2016. Airborne Electromagnetic Surveys for Carrying Out Feasibility Studies for Constructing Road and Rail Tunnels in Himalaya, Journal of Rock Mechanics & Tunnelling Technology (JRMTT), 22 (2), pp 99-112.

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